A Comparative Evaluation of Microleakage of Two Low-Shrinkage Composites with a Conventional Resin Composite: an In Vitro Assessment.

STATEMENT OF THE PROBLEM
Polymerization shrinkage stress in composite restorations may lead to microleakage. Clinical methods such as using low-shrinkage composites have been suggested to overcome this problem; however, there are controversies about their efficiency in decreasing the microleakage.


PURPOSE
This in vitro study was conducted to compare the microleakage of two low-shrinkage resin composites with a conventional one.


MATERIALS AND METHOD
Fifty class V cavities of 2.5×3×2 mm (depth× length× width) were prepared in the buccal surfaces of intact bovine incisor teeth with the incisal margin on the enamel and gingival margin on the cementum. The teeth were randomly divided into 5 groups. In group 1, Clearfil APX (conventional) with SE Bond was used in 2 layers (Kuraray; Japan). In group 2, GC Kalore (low -shrinkage) with GC UniFil Bond was applied in one layer (GC Company). In group 3, the material of group 2 was applied in two layers. In group 4, FiltekP90 (low -shrinkage) with P90 System adhesive was applied in one layer (3M ESPE). In group 5, the materials of group 4 were applied in two layers. The samples were thermocycled and immersed in 0.5% fuchsin solution for 24h. The restorations were sectioned in buccolingual direction. Then they were evaluated for microleakage by using a stereomicroscope and scored as 0, 1, 2, and 3 and then Kruskal-Wallis test was used (p< 0.05).


RESULTS
The groups were not significantly different regarding the microleakage in the coronal and cervical margins (p< 0.423 and p< 0.212, respectively); however, the Filtek P90 yielded the best results. In all groups, except group 5 (p= 0.018), the cervical margins had greater microleakage than the coronal margins.


CONCLUSION
The results suggested that low-shrinkage resin composites may not reduce the marginal microleakage. The proper use of conventional resin composites may offer comparable clinical results.


Introduction
Although resin composites have become one of the clinicians' primary materials of choice for most restorations in recent years, their polymerization contraction is still a fundamental problem. The polymerization con-traction stresses can result in debonding at composite/tooth interface over time. [1] This conflict may also lead to enamel fracture and deflection of cusps. [1][2] Other problems associated with the polymerization contraction of resin composite are postoperative sensitivity, Among the several clinical approaches to reduce this stresses is using low-modulus liners between the tooth and composite which leads to uniform distribution of polymerization stress. Using low-intensity light at the beginning of polymerization can reduce the polymerization speed, hence, the composite is free to flow and less tensile stress is generated. The polymerization shrinkage stresses can also be decreased by applying incremental method for restoration. Reducing the cavity configuration factor (C-factor) is another effective alternative, considering that internal stress increases with higher Cfactor. [1,3] The clinical application of these techniques is not only time-consuming, but the reports regarding their advantages are also controversial. [4][5][6] Beyond the scope of clinical restorative techniques, there are other approaches to reduce the polymerization shrinkage of resin composites such as increasing the filler loading and molecular weight in reactive groups as well as modifying the material formulation. But, in spite of these challenges, polymerization shrinkage has remained an intrinsic property of the resin matrix. Therefore, a modification in resin matrix formulation seems to be the answer to the problem. [7][8] Silorane monomer in Filtek composite (3M ESPE) and DuPont monomer in Kalore composite (GC Company) are examples of newly-marketed resin composite materials. GC Kalore is a kind of low-shrinkage composite with DuPont monomer. It is a methacrylate-base monomer with DX-511 molecule. (Figure 1) Some scientific investigations reported that the silorane-based composites exhibited significantly lower polymerization shrinkage than the conventional methac-rylate-based composites [1,2,4,[6][7] however; others found no significant difference between the two systems. [3,5,8] Due to the noticeable controversies, the objective of the present study was to compare the microleakage of two available low-shrinkage composites, GC Kalore (GC) and Filtek Silorane (3M-ESPE), with a conventional resin composite, Clearfil APX (Kuraray). The null hypothesis stated these two low-shrinkage composites and the conventional composite do not have statistically significant difference in terms of marginal microleakage.

Materials and Method
Fifty extracted intact bovine maxillary incisors, all caries-free and without crack (examined with a light curing-unit), were selected for the study. The tissue debris was removed by using a scaler, the samples were rinsed with water, and then the teeth were kept in normal saline at room temperature until used.
All the teeth were cleaned with Pumice and distilled water. Class V cavities were prepared in labial surfaces by using a high-speed handpiece and diamond fissure burs (008-Diaswiss, Swiss) under water coolant.
Each bur was replaced after five preparations. The entire experiments were performed by the same operator.
The cavities all had similar dimensions (2.5×3×2 mm) as depth×length×width; so that the margins did not pass the mesial and distal line angles. The incisal margins were located 1 mm above the cement enamel junction (CEJ) and gingival margins of the cavity were terminated 1 mm below the CEJ. The limits were initially pencil-marked on the labial surfaces of the teeth. The prepared teeth were randomly divided into 5 groups (n= 10), each to be treated with specific material (Table 1).
In group 1, SE Bond (self-etch system) + Clearfil APX composite (Kuraray Company) was used, and the to restore all preparations.
The restoration procedures in the five groups were performed with small differences. In group 1, two-bottle self-etch SE Bond System was applied according to the manufacturer's recommendations. Self-etch primer was first applied on the walls and margins of all prepared cavities for 30 seconds and air-dried. Then, bonding was applied, air-dried, and finally light-cured for 10 seconds by using Litex 680A unit (Dentamerica; USA). To restore the cavities, Clearfil APX was applied incrementally in horizontal layers (1mm in the first layer and 1.5 mm in the second layer); each layer was light-cured for 40 seconds. In group 2, two-bottle self-etch GC UniFil Bond System and GC-Kalore composite were applied.
The procedures were the same as done in group 1. In group 3, the procedures were similar to group 2, except for the cavity preparations which were restored with one layer and the entire bulk was cured for 40 seconds. In group 4, two-bottle silorane-based self-etch system (P90 Bond System) was applied according to the manufacturer's instructions. First, the walls and margins of all cavi-ties were conditioned by self-etch primer of P90 bonding system, air-dried, and light-cured for 10 seconds.
Then, the bonding was applied, air-dried, and lightcured for 10 seconds. Next, the silorane-based Filtek    ( Table 3) Mann-Whitney test was used to compare the coronal and cervical margin of each group in terms of dye penetration. In this regard, the higher scores were seen in cervical margins; however, significant difference was seen only in group 5 (p= 0.018). (Table 4)

Discussion
Microleakage assessment is among the most common methods of evaluating the quality of dental restoration materials. [3] This article compared the microleakage in a silorane-based composite (Filtek P 90) and two other brands of resin composites, GC Kalore low-shrinkage and conventional Clearfil APX.
Self-etch bonding systems are more hydrophilic compared to total-etch systems. [3] In the current study, in order to eliminate this variation between the two bonding systems and to increase the hydrophilicity of adhesive layers when the samples were exposed to fuchsin solution, only self-etch bonding system was used for all experimental groups.
Compatibility of the adhesive system and the corresponding composite is one of the important clinical features in choosing the materials. [4,6,8] Using silorane-based adhesives is essential when employing silorane-containing composites; [3][4]9] however; the monomer used in GC Kalore composite has been reported to be compatible with other adhesives. [4] Nevertheless, in groups with GC Kalore composite, the adhesive used (GC UniFil bond) was among those suggested by the manufacturer.
In this study, prior to application of self-etch bonding, enamel margins were etched by 37% phosphoric acid for 15 seconds because previous studies showed that etching the enamel was efficient in bonding process and increased the marginal adaptation. [10][11][12] The null hypothesis of this study was that the new composites with low polymerization shrinkage had no difference in microleakage manifestation compared with the conventional ones. The results supported the hypothesis, as there was no statistically significant difference between the groups in enamel and dentin margins.
The microleakage scores in coronal/enamel and cervical/ dentin margins in different groups were not significantly different; yet, the least microleakage score was observed in Filtek P90 groups. The findings of this study were in line with those studies that reported the presence of silorane monomer and ring-opening polymerization reaction as the probable major cause of lower microleakage. [2,[6][7] Other studies reported the silorane-containing composites to have slower initial polymerization reaction. [2,4,6,[13][14]After the beginning of the radiation, polymerization process may last up to 20 minutes and this means that silorane circles continue to open until the conformation of three-dimensional polymer network to occur. [6] This factor may help release of stress from polymerization process and decrease the microleakage.
Studies done by Gao et al., [15] Al-Boni and Raja, [1] Bagis et al., [16] and Umer et al., [3]showed that Filtek The results of this study presented no statistically significant difference between one layer/bulk and double-layer/incremental techniques. It was also reported that when using silorane-based composite system, the configuration of cavity design and polymerization process via light-curing technique were as effective on bond stability as when dimethacrylate-based composite system was used. [4,7] Other studies detected that the incremental technique may provide the same quality in deeper cavity designs when the silorane-containing composites were employed. [6,14] The results of this study do not support the findings reported by Yamazaki et al. [5] The main reasons may be the different composites and fatigue testing methods (thermocycling versus load cycling) used in their study. [5] As it was expected and as reported by previous studies, [22][23] the microleakage scores of each group were greater in the cervical margins than the coronal margins. Except in group 5, no significant difference was observed between the enamel/ coronal and dentin/cervical margins.
The finding of this study revealed no statistically significant difference microleakage among all groups.
We suggest further studies evaluating the influence of sample storage and load cycling on microleakage. The results would not necessarily translate to clinical practice and future studies in the form of clinical trials are required.

Conclusion
Within the limitations of this study, it can be concluded that the microleakage in Filtek P90 and GC Kalore composites were not significantly different from those of the conventional composites. It must be noted that the results of this study can only be reflected to the tested composite materials.